1. Field of the Invention
The present invention relates to a power transmission device which transmits motive power transmitted from a driving element to a driven element.
2. Description of the Related Art
In recent years, as in the case where a robot comes in contact with an obstacle, it is desired that a joint disposed between links of the robot is flexible as a power transmission element in order to prevent the robot from being damaged even in the case where an impact is applied to the robot.
In view of the above circumstances, there is known an actuator with an elastic member disposed between a driving element and a driven element of a robot (U.S. Pat. No. 5,650,704). This actuator prevents an impact from being directly transmitted to the driven element or the driving element by the elasticity of the elastic member even in the case where the impact occurs in the driving element or the driven element. In the case where a joint has flexibility in this manner, a controlled object (for example, the motion of a joint or the like) oscillates more easily by improving a control response. Therefore, the oscillation of the controlled object is suppressed by a feedback control on the basis of information detected by various sensors or the like.
In the case where various sensors or the like are placed in an abnormal state, however, an appropriate feedback control might not be able to be performed and by extension the oscillation might not be able to be suppressed appropriately. Accordingly, it is conceivable to suppress the oscillation by giving viscosity to the joint even in the case where it is impossible to suppress the oscillation by the feedback control.
Meanwhile, in such a case where a precise operation is required, it is preferable that a joint is stiff in some cases. Specifically, if the stiffness of a joint is able to be varied, appropriate control can be performed in various situations. In order to satisfy this requirement, it is conceivable to use a member whose stiffness is variable such as a nonlinear spring as an elastic member.
Generally, however, the following relational expression is already known in a spring-damper system:
                    [                  MATH          ⁢                                          ⁢          1                ]                                                                                  h            ·            ω                    =                      c                          2              ⁢              m                                      ⁢                                  ⁢                              h            ·                                          k                m                                              =                      c                          2              ⁢              m                                                          (        1        )            where h is a damping constant, ω is an angular frequency, k is an elastic coefficient, m is the mass of a load, and C is a viscosity coefficient.
According to the expression (1), supposing that the mass m of a load and the viscosity coefficient C are constant, the damping constant h varies if the elastic coefficient (stiffness) k representing the elasticity of the elastic member is varied. The damping constant h represents a damping rate of vibration in a process in which a vibrating load converges.
The control processing is performed for each previously-determined period. Therefore, the control processing is able to be easily performed if the vibration of a load in a control period at the present time is able to be predicted from the vibration of the load in the previous control period. In other words, the control processing is able to be easily performed when the damping rate (and by extension the damping constant h) is constant than when it is variable. Accordingly, when the damping constant h is constant in the expression (1), the mass m of the load is constant, and therefore it is necessary to vary the viscosity coefficient C according to a change in stiffness k.
In the case where a joint has viscosity as well as elasticity as described above, it is desirable to vary the viscosity coefficient C as well as the stiffness k. Additionally, there is no problem even if the value of the damping constant h fluctuates as long as within the range where the control is able to be easily performed.